code: def __init__(self, fname, custom_handlers={}):
      self._fname = fname
      self._custom_handlers = custom_handlers
      self.read_header()
      self.read_field_descriptors()
      self.read_metadata()
      self.read_allocation_table()
  
file_path: ffcv/ffcv/reader.py
imports: import numpy
  from .utils import decode_null_terminated_string
  from .types import ALLOC_TABLE_TYPE, HeaderType, CURRENT_VERSION, FieldDescType, get_handlers, get_metadata_type
label_desc: Use image node_id 1 to create a new Reader object  with example: obj = Reader(fname, custom_handlers)